<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box {
            width: 50%;
            margin: 50px auto;
            display: flex;
        }

        .box div {

            border: 1px solid red;
        }

        #id1 {
            flex: 1;
        }

        #id2 {
            flex: 2;
        }

        #id3 {
            flex: 3;
        }

    </style>
</head>
<body>

<div class="box">
    <div id="id1">1</div>
    <div id="id2">2</div>
    <div id="id3">3</div>
</div>
</body>
</html>